Regional Aspects of Prolonged Meteorological Droughts over Mexico and Central America

Major prolonged droughts in Mexico during the twentieth century are mainly related to anomalous dry summers, such as those observed in the 1930s, the 1950s, or the 1990s. Droughts in northern Mexico frequently coincide with anomalously wet conditions over Mesoamerica (i.e., southern Mexico and Central America), and vice versa, displaying a dominant “seesaw” structure in persistent precipitation anomalies, mostly in relation to tropical sea surface temperature (SST) anomalies. A warmer North Atlantic Ocean, expressed as a positive phase of Atlantic multidecadal oscillation (AMO), is related to the occurrence of major droughts in North America associated with weaker-than-normal moisture flux into northern Mexico. Drought over northern Mexico may also be related to changes in transient activity in the Caribbean Sea. During the negative phase of the Pacific decadal oscillation (PDO), the Caribbean low-level jet (CLLJ) weakens and easterly wave (EW) activity increases, leading to more tropical convection over Mesoamerica and less moisture flux into northern Mexico. On the other hand, when EW activity is weak over the intra-Americas seas (IAS) (i.e., the Gulf of Mexico and the Caribbean Sea) because of a stronger-than-normal CLLJ, precipitation increases over northern Mexico. Therefore, the interaction between easterly waves and the trade winds over the IAS appears to be crucial to explain the spatial patterns of droughts that have affected Mexico. In addition, low-frequency modulators, such as AMO or PDO, may serve to explain the spatial patterns of severe prolonged droughts in Mexico during the nineteenth century.